Low frequency inverter fed by a high frequency AC current source

ABSTRACT

Apparatus driven by high-frequency AC current source, for driving an electric load with low-frequency AC current, that comprises a current splitting inductor, for generating, from the high-frequency current source, a first and a second high-frequency AC current sources; a rectifier, coupled to the splitting inductor, consisting of rectifying diodes for rectifying the first and second high-frequency current sources, and capacitors, charged by the diodes, the capacitors being corresponding to the first and second DC current sources; a controllable half-bridge commutator having a first and a second control inputs, the commutator being coupled to the DC current sources, for commutating the DC current sources, for allowing to generate, from the DC current sources, the low-frequency AC current required for driving the electric load; and a control circuitry, having a first and a second outputs, the outputs being coupled to the first and second control inputs, respectively, and outputting two complimentary pulse trains, each of which having a frequency being automatically adjusted according to the operating conditions of the electric load, for controlling the switching time of the commutator, thereby causing the commutator to alternately change the direction of the current passing through the electric load.

FIELD OF THE INVENTION

This application is a 371 of PCT/IL03/00074 filed Jan. 30, 2003.

The present invention relates to the field of power switching inverters.More particularly, the present invention relates to a method andapparatus for generating a low frequency AC current for driving linearor nonlinear loads, and in particular for driving arc type lampscommonly known as High Intensity Discharge (HID) lamps.

BACKGROUND OF THE INVENTION

Currently, there are several types of switch mode converters andinverters, which are widely used for DC-to-DC, DC-to-AC, AC-to-DC andAC-to-AC power conversion. Currently, there are loads, the operation ofwhich is optimized and efficiency maximized, if driven by special drivesignal. For example, High Intensity Discharge (HID) lamps need to bedriven by a low frequency AC signal, because high-frequency drive signalmay destabilize the lamp's arc due to existence of acoustic resonance,which is a known phenomenon in the art. Accordingly, an inverter drivingan HID lamp must have a current source nature (as opposed to voltagesource nature) such that its characteristics contribute to the stabilityof the lamp's arc. One way to implement a low-frequency driver is toutilize electromagnetic ballast that is based on a large inductor, whichis placed in series with the power line voltage. An alternative andpreferred approach is to generate the low frequency signal by a switchmode inverter. A typical prior art solution is illustrated in FIG. 1.

FIG. 1 depicts a line rectifier (1), a power factor correction section(PFC), a buck converter that comprises a power switch Q_(B), a stirringdiode D1, an inductor Lf and a filtering capacitor Cf. The buckconverter is controlled to operate as a current source by utilizing afeedback loop (not shown). The controlled DC current (I) is then fed toa commutator that is implemented by a full-bridge inverter (Q1 to Q4),and, therefore the lamp is driven by AC signal. Ignitor 2 is normallyplaced in series with lamp 3, in order to allow providing to the lampthe high-voltage spike that is required for its ignition phase. Thecircuit's configuration shown in FIG. 1 fulfills the lamp requirementsin terms of ignition and low-frequency AC current. However, thisimplementation is rather expensive since it requires 5 power switches(in addition to the switches in the PFC circuitry) and an ignitor.Another drawback of this implementation is, that the power transistorsare ‘hard-switched’ (i.e., they are switched between states while beingunder voltage) and, therefore, will have high switching losses. Theproblem of switching losses associated with the Buck section (i.e., Qb)is acute, because the Buck switch should preferably be operated at ahigh switching frequency. Another drawback associated with theconfiguration of FIG. 1 is that the spike-type ignition voltagerestricts the distance between the ballast and the lamp, because shortpulses, such as an ignition voltage pulse, decay rather fast as afunction of their travel distance.

FIG. 2 (prior art) shows another solution for HID lamp ballast. Lamp 3is driven by a high frequency signal generated by a half-bridge inverter(Q7, Q8). Ignition is accomplished by resonant circuit Cr and Lr. Inorder to ignite the lamp, the half-bridge inverter is driven by afrequency that is slightly higher than the resonant frequency f_(r):

$f_{r} = \frac{1}{2\pi\sqrt{L_{r}C_{r}}}$The resonant circuitry generates a high voltage across Cr, which igniteslamp 3. Once lamp 3 is ignited, the frequency of the drive signal ischanged to f_(s) in order to maintain the required magnitude of thelamp's current. The circuit's configuration shown in FIG. 2 is simplerand less expensive than the circuit's configuration shown in FIG. 1, andthe transistors (i.e., in FIG. 2) are ‘soft-switched’ (i.e., switchedunder zero-voltage condition). However, the configuration shown in FIG.2 suffers from several drawbacks. One drawback is associated withacoustic resonance, which usually develops within the cavity of the lampwhenever HID lamps are driven by high-frequency signals. Acousticresonance normally causes arc instability, rupture/collapse of the arcand even explosion of the lamp. Although several methods have beensuggested to overcome the acoustic resonance problem, for examplefrequency modulation and automatic frequency shifts, none of them hasproven to be efficient for various types of lamps.

According to one aspect, the bridge depicted in FIG. 2, comprising Q5 toQ8, is driven by utilizing a combination of high and low frequencysignals. For example, during the period of the first half of each cycleof the low-frequency signal, Q8 is switched into its conductive state,while Q5 and Q6 are driven by the high-frequency signal. Accordingly,the lamp current originates from Q5 and Q6. During the period of thesecond half of each cycle of the low-frequency signal, Q6 is switchedinto its conductive state, and Q7, Q8 are driven by the high-frequencysignal, causing the lamp current to flow in opposite direction.Therefore, by utilizing this type of control method, the lamp can bedriven, during its normal operating state, by a low frequency current.However, the latter control method has a drawback, being associated withthe complicated control circuitry that is required for suchimplementation. In addition, the latter control method involvesutilizing four power switches, which is another drawback. Furthermore,as would be apparent to a person skilled in the art, the latter powerswitches are switched under hard switching conditions (i.e., switchedunder excessive voltage), thereby causing to significant switchinglosses.

Another major drawback of the ballast shown in FIG. 2 is associated withthe fact that the resonant circuit, used to generate the high voltagefor ignition, is driven by a voltage source (the bus capacitor CBUS).Whenever the drive frequency is close to resonance frequency, theresonant circuitry (i.e., Lr and Cr) introduces essentially a zero ohmicresistance. Consequently, very high currents may develop, which maydamage the apparatus. Accordingly, a special protection circuitry isrequired, that will allow providing to the lamp the high voltage that isrequired for its proper ignition, while guarantying that the resonantcurrent is maintained at safe magnitude.

Another major drawback of the ballast shown in FIG. 2 is associated with‘hot ignition’ phase of the lamp. The practical maximum voltage that isdeveloped by a resonant circuit, such as shown in FIG. 2, (Lr, Cr), isinsufficient for igniting hot HID lamps (i.e., ‘hot ignition’), becausehot ignitions involve delivering to the lamp very high instantaneousvoltages, i.e., between 15 kV and 45 kV. Therefore, an extra ignitor isrequired for generating the high voltage. Such extra ignitors areaffiliated into conventional apparatuses as extra modules, which areplaced in series with the lamp, causing additional complications andcost.

All of the methods described above have not yet provided a simple andefficient way for providing to an HID lamp the optimized power requiredfor its ignition phase, whether ‘cold ignition’ or ‘hot ignition’, aswell as for its normal (i.e., ‘steady-state’) operation.

There is thus a widely recognized need for electronic ballast for HIDlamps that will have less power switches and will produce a lowfrequency AC current to drive the lamp. It would be also desirable thatthe same circuit be capable of producing the high-voltage required forthe lamp's ignition phase, while self-regulating the maximum current ofthe power switches during ignition. It would be also advantageous tohave an apparatus, which would be capable of generating the high voltagethat is required also for hot-ignitions of HID lamps.

It is an object of the present invention to provide a method andapparatus for providing low-frequency AC current to electric loads, suchas HID lamps, with improved efficiency.

It is another object of the present invention to provide a method andapparatus for providing efficient AC ‘cold’/‘hot-ignition’ current thatare required to the operation of electric loads.

It is yet another object of the present invention to providelow-frequency AC current to electric loads, using soft switching.

It is still another object of the present invention to extend thereliability of electric loads, such as HID lamps.

Other objects and advantages of the invention will become apparent asthe description proceeds.

SUMMARY OF THE INVENTION

The present invention is directed to a high frequency AC current sourcedriven inverter, for providing, to an electric load, a low frequency ACcurrent. The inverter includes a current splitting inductor, forsplitting the high frequency AC current source into two high frequencyAC current sources, a rectifier, for generating two DC current sources,by rectifying the resulted two high frequency current sources and acommutator, for generating a low frequency AC current from the resultedDC current sources. In some cases, loads, for example an HID lamp, mayrequire an iginition phase. Accordingly, the inverter may also include aresonant circuitry, for generating a high voltage that is needed forignition of such loads. In addition, a very high voltage spiker may beincluded in the low frequency inverter, which is fed from theabove-mentioned resonant circuitry, for ‘hot’ ignition of an HID lamp.An additional feature of present invention is that all power switchesincluded in the inverter are soft switched in order to essentiallyeliminate switching losses.

Preferably, the inverter comprises:

-   a) a current splitting inductor, for generating a first and a second    high-frequency AC current sources;-   b) a rectifier, coupled to the splitting inductor and consisting of    rectifying diodes. The rectifier is utilized for rectifying the    first and second high-frequency current sources. In addition, the    rectifier may include two capacitors, in order them to be charged by    the rectifier's diodes. These capacitors may be utilized as    corresponding first and second DC current sources;-   c) a controllable half-bridge commutator having first and second    control inputs. The commutator may be coupled to the DC current    sources, in order to commutate them, for allowing to generate, from    the DC current sources, the low-frequency AC current that is    required for driving the electric load; and-   d) control circuitry, having first and second outputs. The outputs    may be coupled to the first and second control inputs, respectively,    and may output two complimentary pulse trains, each of which having    a frequency that is automatically adjusted according to the    operating condition of the electric load, for controlling the    switching time of the commutator, thereby causing said commutator to    alternately change the direction of the current passing through the    said electric load.

The electric load might be a High Intensity Discharge (HID) lamp, or anelectric motor. In the latter case, the torque and rotating speed of themotor will be controlled by the magnitude of the current and by theswitching frequency of the commutator, respectively.

According to one embodiment of the present invention, the rectifier isimplemented by utilizing diodes in a ‘full-bridge’ or ‘half-bridge’configuration and the half-bridge commutator is implemented by utilizingfirst and second controllable switching means, which may be alternatelyswitched from conductive state to non-conductive state. According to oneaspect, the controllable switching means are transistors.

According to one aspect, the inverter may include a resonant ignitioncircuit, for generating the voltage required for ‘cold-ignition’ of theHID lamp.

Preferably, the resonant ignition circuit may comprise:

-   a) a capacitance, which is coupled in parallel to the HID lamp; and-   b) an inductor, which is connected in series with respect to the    lamp, and form with the capacitance a serial resonant circuitry. The    resonance frequency of the serial resonance circuitry is selected to    be higher than the operating frequency of the current passing    through the HID lamp.

According to another aspect, the inverter may further comprise anignition circuitry, for generating the high voltage required for‘hot-ignition’ of the HID lamp.

Preferably the ignition circuitry may comprise:

-   a) an autotransformer (Lr), having one of its portions connected in    series with the resonant ignition circuitry, The inductor of the    resonant ignition circuitry is the secondary side of a transformer    and part of the resonant ignition circuitry. The primary side of the    transformer has a first contact that is coupled to a first end of a    capacitor. According to one aspect, the autotransformer is    implemented by a transformer having first and second windings. The    first and second windings are utilized, in this case, as the first    and second portions, respectively;-   b) a spark gap (SPRK), having one of its ends coupled to a second    end of the primary side. The second end of the SPRK may be coupled    to a second end of the capacitor. The SPRK introduces a high    impedance (essentially infinite) whenever the voltage across it is    lower than a predetermined value (commonly referred to as    ‘breakdown’ value), and a momentarily low (essentially zero)    impedance whenever the voltage across it exceeds said ‘breakdown’    value; and-   c) a rectifier, which may be fed by a second portion of the    autotransformer, for allowing the energy, required for hot-ignition,    to be stored in the capacitor. The stored energy may be forwarded to    the secondary side of the transformer, whenever said SPRK introduces    a low impedance, thereby allowing to obtain the voltage required for    hot-ignition of the lamp. According to one aspect, the rectifier is    a voltage doubler.

The operating condition, under which the lamp may operate, is the cold,or hot, ignition phase, in which case the frequency of the pulse trainsis close to the resonance frequency of the Resonant Ignition circuitry,or an intermediate phase, in which case the frequency of the pulsetrains gradually decreases (sweeps), or the normal state, in which casethe frequency of the pulse trains is relatively low, and essentiallyconstant.

According to one aspect, the current splitting inductor is implementedby an autotransformer, for allowing utilizing a relatively low ACvoltage source, or, according to another aspect, the current splittinginductor is implemented by a transformer, for allowing isolation betweenthe signal source side and the load side.

According to one aspect, the high-frequency AC current source isimplemented by utilizing a high-frequency half-bridge inverter, which isplaced between a DC voltage source and the current splitting inductor.

Preferably, the high-frequency half-bridge inverter comprises:

-   a) a capacitor, having its first contact coupled to an input contact    of the current splitting inductor, for blocking DC signals;-   b) an inductor, having its first contact coupled to a second contact    of the capacitor, for limiting the input current of said current    splitting inductor; and-   c) a third and a forth controllable switching means (Q11, Q12),    which are coupled to each other by their corresponding first    contact, and to the DC voltage source by their corresponding second    contact. The first contacts of the controllable switching means are    coupled to a second contact of the inductor, for allowing generating    the high frequency of the AC current source. The high frequency of    the AC current source is essentially higher than a resonance    frequency caused by the capacitor and the inductor, for allowing    soft-switching the third and forth controllable switches.

According to one aspect, the high-frequency AC current source isimplemented by utilizing a Current-Sourcing Push-Pull Parallel ResonantInverter (CS-PPRI), which is placed between a DC voltage source and thecurrent splitting inductor.

Preferably, the Current-Sourcing Push-Pull Parallel Resonant Inverter(CS-PPRI) comprises:

-   a) a transformer, the primary side of which including first and    second input inductors. The secondary side of the transformer is    utilized as the current splitting inductor;-   b) a first Inductor (Lc), having its first contact coupled to a    first contact of the first input inductor, and its second contact    coupled to a first contact of the second input inductor;-   c) a resonant Capacitor (Cc), having its first contact coupled to a    second contact of the first input inductor, and its second contact    coupled to a second contact of the second input inductor. The    resonant capacitor (Cc), the first input Inductor (Lc) and the input    inductors form a Parallel Resonant Circuitry (PRC), for allowing    generating an alternating current source;-   d) a second Inductor (Lin), having a first contact that could be    connected to a DC power source and a second that may be connected to    a middle contact of said first Inductor (Lc). The inductance of the    second Inductor (Lin) is larger than the inductance of the first    Inductor (Lc), for allowing the second Inductor (Lin) to generate    the current required for driving the PRC;-   e) a first controllable switch (Q12), having its first contact    coupled to the first contact of the capacitor, and its second    contact coupled to ground;-   f) a second controllable switch (Q13), having its first contact    coupled to the second contact of the capacitor, and its second    contact coupled to the ground; and-   g) a Soft Switching Controller (SSC), for soft switching the second    and third switches (Q12, Q13). The input of the SSC is fed with a    signal that represents the instantaneous magnitude of the signal at    the second contact of the second Inductor (Lin), and the SSC    generates two complementary trains of digital signal. One of the    trains may be fed to an input terminal of the second switch (Q12)    and the second train may be fed to an input terminal of the third    switch (Q13). The trains cause the corresponding switches to    alternately switch from conductive to non-conductive state, in    synchronization with the instants at which the instantaneous    magnitude reaches essentially a zero value. Only one switch may be    in its conductive state at a given time.

According to one aspect, the high-frequency AC current source isimplemented by utilizing an input circuitry in a Flyback configuration.The input circuitry is placed between a DC voltage source and thecurrent splitting inductor.

Preferably, the Flyback configuration comprises:

-   a) a transformer, the primary side of which is an input inductor    (L1). The input inductor could be connected, by one of its contacts,    to a DC power source. The secondary side of the transformer is the    current splitting inductor; and-   b) a controllable switch (Q14), having its first contact coupled to    a second contact of the input inductor (L1). The second contact of    the controllable switch may be coupled, via a resistor, to ground.    Whenever the controllable switch is in its conductive state, it    causes the input inductor (L1) to store energy, and, whenever the    controllable switch is in its non-conductive state, at least some of    the stored energy is forwarded to the current splitting inductor.

The high-frequency half-bridge inverter, Current-Sourcing Push-PullParallel Resonant Inverter (CS-PPRI) and the Flyback configuration mayfurther include a current feedback circuitry, for controlling thecurrent passing through the HID lamp.

Preferably, the current feedback circuitry comprises:

-   a) first and second windings of a current transformer. Each of the    windings may be connected in series with the corresponding first and    second high-frequency current sources, for sampling the current    passing through the corresponding current source;-   b) a rectifier, for generating a first signal that represents the    rectified sampled currents;-   c) a first amplifier, having at least one reference input, which is    connected to a constant reference value. The first amplifier also    has a signal input, to which the first signal is forwarded, for    generating an error signal that represents the deviation of the    first signal from the reference value; and-   d) a current mode PWM (Pulse Width Modulation) modulator, having a    first input, to which the error signal is forwarded, and a second    input, to which a second signal, representing the current of the    high-frequency AC current source, is fed. The PWM modulator also has    at least one output, for outputting a corresponding train of pulses,    the duty-cycle of which is a function of the error signal and of the    second signal, and is connected to the corresponding driver, the    output of which is coupled to the corresponding controllable switch,    for controlling its switching time, for causing the current passing    through the HID lamp to be at the required value, thereby completing    the feedback.

Alternately, the PWM modulator may be a voltage mode PWM controller, inwhich case the second input accepts a periodical ramp as a referencesignal. The parameters of the periodical ramp, being at least the cycleduration and ramp's slope, could be determined so as to optimize theoperation of the apparatus.

The inverter may further include a voltage feedback circuitry, forallowing clamping the voltage across the HID lamp, whenever the lamp isin its “off” state, and increasing the current of the lamp during its‘warm-up’ period.

Preferably, the voltage feedback circuitry comprises:

-   a) a sampling circuitry, for sampling a voltage that represents the    voltage across the lamp;-   b) a second amplifier, having an input, to which the sampled voltage    is forwarded, for generating a third signal. The third signal is    added to the first signal and is essentially zero whenever the lamp    is in its ignition phase, for allowing providing, to the lamp, a    relatively increased current, while the lamp is in its ‘warm-up’    stage and the voltage across it is relatively low. The third signal    is essentially proportional to the voltage across the lamp while the    lamp is in its normal operating state, for allowing to decrease the    (increased) current to the required operating value; and-   c) a third amplifier, having an input, to which the voltage    representing the voltage across the lamp is forwarded, for    generating a fourth signal. The fourth signal is forwarded to the    first amplifier and is essentially large whenever the lamp is in its    ‘off’—state, or there is no lamp connected to the apparatus, for    allowing clamping the voltage on the lamp to a safe level. The    fourth signal is essentially zero while the lamp is in its ignition    phase or in its normal operating state, for allowing the lamp's    current to reach the required operating value.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other characteristics and advantages of the invention willbe better understood through the following illustrative andnon-limitative detailed description of preferred embodiments thereof,with reference to the appended drawings, wherein:

FIG. 1 (prior art) illustrates a low-frequency electronic ballast forHID lamps;

FIG. 2 (prior art) illustrates a high-frequency electronic ballast forHID lamps;

FIG. 3 illustrates two-transistor inverter and resonant ignitor,according to a preferred embodiment of the present invention;

FIG. 4 illustrates a typical drive sequence for the circuit of FIG. 2,which includes ignition phase, frequency sweep and normal operatingfrequency;

FIG. 5 depicts a part of the circuit of FIG. 3, illustrating the currentpaths when Q10 is conducting and the input current Iin is positive,according to a preferred embodiment of the present invention;

FIG. 6 depicts a part of the circuit of FIG. 3, illustrating the currentpaths when Q10 is conducting and the input current Iin is negative,according to a preferred embodiment of the present invention;

FIG. 7 depicts a part of the circuit of FIG. 3, illustrating the currentpaths when Q9 is conducting and the input current Iin is positive,according to a preferred embodiment of the present invention;

FIG. 8 depicts a part of the circuit of FIG. 3, illustrating the currentpaths when Q9 is conducting and the input current Iin is negative,according to a preferred embodiment of the present invention;

FIG. 9 illustrates implementing the circuit of FIG. 3 with anautotransformer, according to a second embodiment of the presentinvention;

FIG. 10 illustrates implementing the circuit of FIG. 3 with atransformer, according to a second embodiment of the present invention;

FIG. 11 illustrates half-bridge realization of the current source Idshown in FIG. 3, according to a preferred embodiment of the presentinvention;

FIG. 12 illustrates sourcing push pull resonant inverter realization ofthe current source Id shown in FIG. 3, according to a preferredembodiment of the present invention;

FIG. 13 illustrates a flyback realization of the current source Id shownin FIG. 3, according to a preferred embodiment of the present invention;

FIG. 14 illustrates a control circuit of the circuit shown in FIG. 13,according to a preferred embodiment of the present invention;

FIG. 15 illustrates incorporating a very high voltage spiker to the lowfrequency generator, according to a preferred embodiment of the presentinvention; and

FIG. 16 illustrates typical waveforms of signals associated with thecircuit shown in FIG. 3.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is directed to a low frequency inverter fed by ahigh frequency alternating current.

FIG. 3 illustrates a two-transistor inverter and resonant ignitor,according to a preferred embodiment of the present invention. T1 is anautotransformer that is utilized as a current splitting inductor. T1 isdriven by a high frequency current source Id. Inductor T1 is split intotwo portions, thereby forming two corresponding high frequency ACcurrent sources. The two high frequency AC current sources are followedby a ‘full-wave’ rectifier, comprising rectifying diodes D1 to D4 thatrectify the corresponding AC current sources, and two capacitors (i.e.,C1 and C2). The resulted rectified (DC) currents (i.e., generated by C1and C2) are then forwarded to a controllable half-bridge commutator,which comprises controllable power switches. In FIG. 3, the controllablepower switches are power transistors Q9 and Q10. The DC current sourcesare commutated, by the commutator, for allowing to generate, from theresulted DC current sources, the low-frequency AC current required fordriving the electric load. The rectifier could be implemented by a‘half-bridge’ configuration, as illustrated in FIGS. 13 and 14.

A control circuitry (not shown), having two output, is utilized forcontrolling the operation of the controllable commutator (Q9, Q10), byproviding to the commutator's inputs (i.e., 31 and 32) two complimentarypulse trains. The frequency of the pulse trains is automaticallyadjusted according to the conditions (i.e., ‘cold’/‘hot’ ignition,intermediate state, normal operating state) of the electric load, inorder to control the switching time of the commutator, thereby causingthe commutator to alternately change the direction of the currentpassing through the electric load, which might be a High IntensityDischarge (HID) lamp. Additionally, a current and/or voltage feedbackcircuitry could be utilized, for controlling the current andcontrolling, or clamping, the voltage across the load. An exemplaryfeedback circuitry is shown in FIG. 14.

The current passing through the lamp (when ignited) is a symmetricalsquare wave AC current. Under normal operating conditions (i.e., in asteady state), the drive frequency of Q9 and Q10 is kept low in order tomaintain the lamp under safe operating condition. During the ignitionphase, Q9 and Q10 are driven by a relatively high frequency signal,i.e., just above the resonance frequency as determined by the serialresonant ignition circuitry that comprises Cr and Lr. The latter signalgenerates a high voltage across Cr, which is connected in parallel tothe lamp, the power of which is sufficient for cold ignition of the HIDlamp. An optional drive sequence, for driving the inverter stage of FIG.3, is shown in FIG. 4.

Of course, the autotransformer T1 shown in FIG. 3 could be replaced by atransformer, as shown in FIGS. 10 to 14, for providing isolation betweenthe signal source side and the load side. In addition, the highfrequency current source could be implemented by utilizing a highfrequency half-bridge inverter, such as the inverter shown in FIG. 11,or, alternately, by utilizing a Current-Sourcing Push-Pull ParallelResonant Inverter (CS-PPRI), such as the inverter shown in FIG. 12, or,alternately, by utilizing an input circuitry in a Flyback configuration,such as the inverter shown in FIGS. 13 and 14.

In FIG. 4, during ignition phase, the frequency of the driving pulses isclose to the resonance frequency of Lr, Cr. After the lamp is ignited,the frequency of the driving pulses gradually decreases (‘sweeps’) untilthe nominal low operating frequency is reached. As is apparent in FIG.4, whenever Q9 is in its conductive state (i.e., Vgs9 is at binary“high”) state), Q10 is in its non-conductive state (i.e., Vgs10 is atbinary “low” state), and vise versa. Additionally, transistor Q9 is atits conducting state when the input current is positive, as well as whenthe input current is negative (i.e., at different periods). Likewise,transistor Q10 is at its conducting state when the input current ispositive, as well as when the input current is negative (i.e., atdifferent time). Obviously, Q9 and Q10 are not in their conductive stateat the same time. Accordingly, and in order to facilitate theunderstanding of its operation, the circuit of FIG. 3 is functionally‘split’ into the corresponding parts, as described in FIGS. 5 to 8.

FIGS. 5 and 6 illustrate the current path when Q10 is in its conductivestate, while Q9 is in its non-conductive state, and the input currentIin is rectified by a full-bridge comprising diodes D4 and D2,respectively, according to a preferred embodiment of the presentinvention. I17 is associated with the positive cycles of input currentIin, and I21 is associated with the negative cycles of input currentIin. I17 and I21 have the same first direction with respect to lampLamp.

FIGS. 7 and 8 illustrate the current path when Q9 is in its conductivestate, while Q10 is in its non-conductive state, and the input currentIin is rectified by a full-bridge comprising diodes D1 and D3,respectively, according to a preferred embodiment of the presentinvention. I3 is associated with the positive cycles of input currentIin, and I10 is associated with the negative cycles of input currentIin. I3 and I10 have the same second direction with respect to lampLamp, which differs from the first current(s) direction, therebyobtaining the effect of alternating current that passes through thelamp, as required. In addition, the operating frequency of the lamp isthe switching frequency at which Q9 and Q10 are switched from conductivestate to non-conductive state, and the switching frequency is relativelylow. The resulted waveforms of the signals associated with the circuitshown in FIGS. 5 to 8 are depicted in FIG. 16.

The low frequency inverter shown in FIG. 3 has at least the followingadvantages:

-   a) The size of transformer T1 is relatively small due to the high    frequency condition under which it operates.-   b) the current passing through the load (i.e., Lamp) is a low    frequency AC current.-   c) generating the required ignition voltage is obtained by adding    resonant network (i.e., Lr and Cr), and by driving the corresponding    half-bridge diodes with a signal having a frequency that is slightly    above the frequency of said resonant network.-   d) the current through the switches (i.e., Q9 and Q10) is limited to    the input current Id. Therefore, no excessive current is developed    in the circuit, in contradiction to the prior art configurations.

FIG. 9 illustrates implementing the circuit of FIG. 3 with anautotransformer, according to one aspect of the present invention.Transformer T1 shown in FIG. 3 has been replaced by autotransformer T1in FIG. 9, to allow utilizing a relatively low input voltage (i.e.,V_(BUS)).

FIG. 10 illustrates implementing the circuit of FIG. 3 with atransformer, according to a third aspect of the present invention.Transformer T1 shown in FIG. 3 has been replaced by transformer T1 inFIG. 10, which is built as a multi-winding magnetic element allowinggalvanic isolation between the input side and the output side.

FIG. 11 illustrates half-bridge realization of the high frequencycurrent source Id shown in FIG. 3, according to another aspect of thepresent invention. The current driver comprises a high frequencyhalf-bridge driver (Q11, Q12). Ls limits the current and Cs blocks theDC component of the current. As may be appreciated by those skilled inthe art, Q11 and Q12 may be ‘soft-switched’ by operating the half-bridgedriver at a frequency that is slightly above the resonant frequency ofLs, Cs, and by employing a zero-cross based controller.

FIG. 12 illustrates Current-Sourcing Push-Pull Parallel ResonantInverter (CS-PPRI) realization of the current source Id shown in FIG. 3,according to a another embodiment of the present invention. The CS-PPRIcircuit is particularly relevant in cases wherein low input voltage isrequired, such as in cases wherein batteries are utilized for poweringthe apparatus. Accordingly, a CS-PPRI-based apparatus will beparticularly useful for powering HID headlight lamps in automotiveapplications.

Switches Q12 and Q13 are controlled by a Soft Switching Controller (notshown), which accepts at its input a signal (not shown) which representsthe instantaneous magnitude of the signal at the middle point of Lc, andgenerates two complimentary trains (not shown) of digital signals (i.e.,pulses), each of which is fed to the corresponding switch (Q12, Q13).These trains cause the switches Q12, Q13 to alternately switch fromtheir conductive state to their non-conductive state in synchronizationwith the instants at which the instantaneous magnitude reachesessentially a zero value. Only one switch could be in its conductivestate at a given time.

FIG. 13 illustrates a flyback realization of the current source Id shownin FIG. 3, according to a preferred embodiment of the present invention.The current splitting element is a coupled inductor (L1) that is part ofa flyback topology. Whenever Q14 is in its conductive state, energy isstored in L1 and, whenever Q14 is in its non-conductive state, L1discharges at least some of its stored energy into the load. Currentwill flow via D5 or D6, depending on which transistor (i.e., Q9 or Q10,respectively) is in its conductive state.

FIG. 14 illustrates a control circuit for the circuit shown in FIG. 13,according to a preferred embodiment of the present invention. D1 and D2are utilized for ‘half-bridging’ the current passing through Tc1 andTc2, respectively, and, C1 and C2 are utilized for filtering out thecorresponding high-frequency components and obtaining Va and Vb,respectively, which are essentially Direct-Current (DC) voltages.Switching Q9 to its conductive state (and Q10 to its non-conductivestate) results in a current passing through the lamp in one direction(i.e., I1). Switching Q10 to its conductive state (and Q9 to itsnon-conductive state) results in a current passing through the lamp inthe opposite direction (i.e., I2). Consequently, the current passingthrough the lamp is an alternating pulsed current, the frequency ofwhich is the switching frequency of Q9 and Q10, which is relatively lowfrequency. Tc1 and Tc2 are utilized for allowing measuring thecorresponding instantaneous currents passing through D1 and D2, whichreflect the corresponding currents consumed by the lamp.

The control circuit, comprising full-bridge rectifier 1401, Q15, Q16, R1to R7, amplifier 1402, PWM modulator 1403 and driver 1405, accepts threetypes of feedback signals, which are summed-up by R6, R7 and R5. Thesummation signal (1402 a) is forwarded to amplifier 1402, which isutilized as comparator, for outputting error signal 1402 b that isassociated with the comparison result between the latter signal (i.e.,1402 a) and a predetermined reference value Vref. R6 forwards (i.e., toamplifier 1402) a feedback signal that is associated with the currentconsumption of the lamp. R7 forwards a feedback signal that isassociated with the maximum allowable voltage across the lamp duringignition phase (Vmax), and R5 forwards a feedback signal that isassociated with the warm-up phase of the lamp.

In the ignition phase, or whenever there is no lamp connected to theapparatus, a relatively high voltage tends to develop. However, it isrequired to limit the allowable maximum voltage in order to maintain theapparatus under safe operating conditions. Accordingly, Q15 is utilizedto limit the output voltage Vbus2. Whenever Vmax increases, theconductivity of Q15 increases, resulting in increased voltage beingforwarded to amplifier 1402 via R7. Accordingly, the output currentdecreases to the (required) predetermined value, and the output voltageVmax is thereby clamped to a safe level.

After the lamp ignites, a warm-up phase takes place, during which thevoltage across the lamp is relatively low, in which case Von is alsolow, causing to Q16 to be in its non-conductive state. Consequently, arelatively high current is provided to the lamp, which complies with therequired current during warm-up phase. The voltage across the lampincreases as the lamp becomes hotter, causing an increase in Von.Therefore, the conductivity of Q16 increases, causing an increase in thecorresponding feedback that is forwarded to amplifier 1402, resulting inlowering the lamp's current to the desire normal operating level.

Error signal 1402 b is forwarded to Current Mode Pulse Width Modulation(CMPWM) 1403, which accepts an additional signal, being representativeof the current passing through the Flyback inductor L1 (i.e., thecurrent of the high frequency AC current source). The task of CMPWM 1403is to maintain the output current (which is the precursor of the currentof the lamp) at the desired level, by adjusting the duty cycle of thedriving signal 1406. The larger the error signal (1402 b), the smallerthe duty cycle, i.e., the smaller the periods at which Q14 is in itsconductive state (and the smaller the energy that is forwarded to thelamp). Of course, the CMPWM could be replaced by a corresponding VoltageMode Pulse Width Modulation (VMPWM), in which case the VMPWM wouldaccept, as the additional signal, a signal that would represent thevoltage across inductor L1.

As a lamp ages, the voltage across it increases. Providing a constantcurrent to the lamp, regardless of the aging phenomenon, will cause thelamp to be overdriven. However, the apparatus shown in FIG. 14 solvesthis situation by increasing the current passing through R5 in responseto the increased voltage across the lamp. A corresponding change inerror signal 1402 b will cause a corresponding decrease in the lamp'scurrent.

Of course, the current feedback circuitry shown in FIG. 14 could beutilized by at least the configurations shown in FIGS. 11 and 12.Accordingly, each controllable switch, which is utilized for controllingthe current of the high frequency AC source, is assigned a correspondingPWM output.

FIG. 15 illustrates incorporating a very high voltage ignitor to the lowfrequency inverter, according to one aspect of the present invention.The circuit generates a very high-voltage spike that is required for hotignition (i.e., restart) of the lamp. The latter spike is obtained by anignition circuitry that comprises Lr, which is utilized as anautotransformer for providing a high voltage, a rectifier (D7, D8, Cp,Cg) to which the high voltage is forwarded, and a spark gap SPRK, onwhich the resulted high voltage is employed, for causing it to introducelow impedance. Spark gap SPRK is essentially a voltage-dependent‘On-Off’ switch, that is characterized by having a very high(essentially infinite) impedance whenever the voltage across it is lowerthan a predetermined value (commonly referred to as a ‘breakdownvoltage’), and very low (essentially zero) impedance whenever thevoltage across it momentarily exceeds said breakdown voltage.

Accordingly, whenever the voltage across Cg reaches the breakdownvoltage of the spark gap, the spark gap ‘collapses’ (i.e., its impedancebeing very low), causing the voltage across Cg to be forwarded to theprimary side of pulse transformer T2. By choosing the proper turns-ratioof T2, the voltage-pulse fed to the lamp can be such that it complieswith the required hot-start conditions. After completion of thehot-start phase, the operating frequency of the commutator (i.e., Q9 andQ10) is decreased, resulting in a decreased voltage, which is notsufficient for activating spark gap SPRK. Therefore, the spark gapremains, during the lamp's normal operation, in its “open” state, anddisconnects the primary side of transformer T2.

Of course, the rectifier (D7, D8, Cp, Cg) may be implemented by a‘voltage doubler’, and the autotransformer by a two-winding transformer,both are features that are known to those skilled in the art.

The above examples and description have of course been provided only forthe purpose of illustration, and are not intended to limit the inventionin any way. As will be appreciated by the skilled person, the inventioncan be carried out in a great variety of ways, employing more than onetechnique from those described above, all without exceeding the scope ofthe invention.

1. Apparatus driven by high-frequency AC current source, for driving anelectric load with low-frequency AC current, comprising: a) a currentsplitting inductor, for generating, from said high-frequency currentsource, a first and a second high-frequency AC current sources; b) arectifier, coupled to said splitting inductor, consisting of rectifyingdiodes for rectifying said first and second high-frequency currentsources, and capacitors, charged by said diodes, said capacitors beingcorresponding to a first and second DC current sources; c) acontrollable half-bridge commutator having a first and a second controlinputs, said commutator being coupled to said DC current sources, forcommutating said DC current sources, for allowing to generate, from saidDC current sources, the low-frequency AC current required for drivingsaid electric load; and d) a control circuitry, having a first and asecond outputs, said outputs being coupled to said first and secondcontrol inputs, respectively, and outputting two complimentary pulsetrains, each of which having a frequency being automatically adjustedaccording to the operating conditions of said electric load, forcontrolling the switching time of said commutator, thereby causing saidcommutator to alternately change the direction of the current passingthrough said electric load.
 2. Apparatus according to claim 1, in whichthe electric load is a High Intensity Discharge (HID) lamp, or anelectric motor, the torque and rotating speed of which are controlled bythe magnitude of the low-frequency AC current and by the switchingfrequency of the commutator, respectively.
 3. Apparatus according toclaim 2, further including a resonant ignition circuit, for generatingthe voltage required for cold-ignition of the HID lamp, comprising: a) acapacitance, being coupled in parallel to the HID lamp; and b) aninductor, being connected in series with respect to the lamp, saidinductor forming a serial resonant circuit with said capacitor, whereinthe resonant frequency of said serial resonant circuit is selected to behigher than the operating frequency of the current passing through saidHID lamp.
 4. Apparatus according to claim 2, further including anignition circuitry, for generating the high voltage required forhot-ignition of the HID lamp, comprising: a) an autotransformer (Lr),one portion of which being connected in series with said resonantignition circuitry, the inductor of which being the secondary side of atransformer and part of said resonant ignition circuitry, the primaryside of which having a first end coupled to a first end of a capacitor;b) a spark gap (SPRK), one end of which being coupled to a second end ofsaid primary side, and a second end of which being coupled to a secondend of said capacitor, said SPRK introduces a high impedance wheneverthe voltage across it is lower than a predetermined breakdown value, anda momentarily low impedance whenever the voltage across it exceeds saidbreakdown value; and c) a rectifier, being fed by a second portion ofsaid autotransformer, for allowing the energy, required forhot-ignition, to be stored in said capacitor, said energy beingforwarded to said secondary side, whenever said SPRK introduces a lowimpedance, thereby allowing to obtain the voltage required forhot-ignition of said lamp.
 5. Apparatus according to claim 4, in whichthe autotransformer is implemented by a transformer having first andsecond windings, being the first and second portions, respectively. 6.Apparatus according to claim 4, in which the rectifier is a voltagedoubler.
 7. Apparatus according to claim 2, wherein the operatingcondition is the cold, or hot, ignition phase, during which thefrequency of the pulse trains is close to the resonance frequency of theResonant Ignition circuitry, or an intermediate phase, during which thefrequency of the pulse trains gradually decreases, or the normal state,during which the frequency of the pulse trains is relatively low, andessentially constant.
 8. Apparatus according to claim 2, furtherincluding a current feedback circuitry, for controlling the currentpassing through the HID lamp, comprising: a) first and second windingsof a current transformer, each of which being connected in series withthe corresponding first and second high-frequency current sources, forsampling the current passing through the corresponding current source;b) a rectifier, for generating a first signal being representative ofthe rectified sampled currents; c) a first amplifier, having at leastone reference input, being connected to a constant reference value, anda signal input, to which said first signal is forwarded, for generatingan error signal representing of the deviation of said first signal fromsaid reference value; and d) a current mode PWM modulator, having afirst input, to which said error signal is forwarded, a second input, towhich a second signal, representing the current of the high-frequency ACcurrent source, is fed, and at least one output, for outputting acorresponding train of pulses, the duty-cycle of which is a function ofsaid error signal and of said second signal, and being connected to acorresponding driver, the output of which being coupled to thecorresponding controllable switch, for controlling its switching time,for causing the current passing through the HID lamp to be at therequired value, thereby completing said feedback.
 9. Apparatus accordingto claim 8, in which the PWM modulator is a voltage mode PWM controller,and the second input accepts a periodical ramp signal as a referencesignal, the parameters of said periodical ramp signal being at least thecycle duration and ramp's slope and being determined so as to optimizethe operation of said apparatus.
 10. Apparatus according to claim 2,further including a voltage feedback circuitry, for allowing clampingthe voltage across the HID lamp, whenever said lamp is in its off state,and increasing the current of said lamp during its warm-up period,comprising: a) a sampling circuitry, for sampling a voltage representingthe voltage across said lamp; b) a second amplifier, having an input, towhich the sampled voltage is forwarded, for generating a third signal,to be added to the first signal and being essentially zero whenever saidlamp is in its ignition phase, for allowing to provide, to said lamp, arelatively increased current, while said lamp being in its warm-up stageand the voltage across it being relatively low, said third signal beingessentially proportional to the voltage across said lamp while said lampbeing in its normal operating state, for allowing to decrease saidincreased current to the required operating value; and c) a thirdamplifier, having an input, to which the voltage representing thevoltage across said lamp is forwarded, for generating a fourth signal,said fourth signal being forwarded to the first amplifier and beingessentially large whenever said lamp is in its off state, or there is nolamp connected to the apparatus, for allowing to clamp the voltage onsaid lamp to a safe level, said fourth signal being essentially zerowhile said lamp being in its ignition phase or in its normal operatingstate, for allowing the lamp's current to reach the required operatingvalue.
 11. Apparatus according to claim 1, in which the rectifier isimplemented by utilizing diodes in a full-bridge or half-bridgeconfiguration.
 12. Apparatus according to claim 1, in which thehalf-bridge commutator is implemented by utilizing a first and a secondcontrollable switching means, said switching means being, wheneverdesired, alternately switched from conductive state to non-conductivestate.
 13. Apparatus according to claim 12, in which the first andsecond controllable switching means are transistors.
 14. Apparatusaccording to claim 1, wherein the current splitting inductor isimplemented by an autotransformer, thereby allowing utilizing arelatively low AC voltage source.
 15. Apparatus according to claim 1,wherein the current splitting inductor is implemented by a transformer,for allowing isolation between the signal source side and the load side.16. Apparatus according to claim 1, in which the high-frequency ACcurrent source is implemented by utilizing a high-frequency half-bridgeinverter, being placed between a DC voltage source and the currentsplitting inductor, comprising: a) a capacitor, a first contact of whichbeing coupled to an input contact of the current splitting inductor, forblocking DC signals; b) an inductor, a first contact of which beingcoupled to a second contact of said capacitor, for limiting the inputcurrent of said current splitting inductor; and c) a third and a fourthcontrollable switching means (Q11, Q12), being coupled to each other bytheir corresponding first contact, and to said DC voltage source bytheir corresponding second contact, said first contact being coupled toa second contact of said inductor, for allowing generating thehigh-frequency of said AC current source, said high-frequency beingessentially higher than a resonance frequency caused by said capacitorand said inductor, for allowing soft-switching said third and forthfourth controllable switches.
 17. Apparatus according to claim 1, inwhich the high-frequency AC current source is implemented by utilizing aCurrent-Sourcing Push-Pull Parallel Resonant Inverter (CS-PPRI), beingplaced between a DC voltage source and the current splitting inductor,comprising: a) a transformer, the primary side of which having a firstand a second input inductors, and the secondary side of which being thecurrent splitting inductor; b) a first Inductor (Lc), a first contact ofwhich being coupled to a first contact of said first input inductor, anda second contact of which being coupled to a first contact of saidsecond input inductor; c) a resonant Capacitor (Cc), a first contact ofwhich being coupled to a second contact of said first input inductor,and a second contact of which being coupled to a second contact of saidsecond input inductor, said resonant capacitor, first Inductor (Lc) andinput inductors forming a Parallel Resonant Circuitry (PRC), forallowing generating an alternating current source; d) a second Inductor(Lin), a first contact of which could be connected to a DC power sourceand a second contact of which being connected to a middle contact ofsaid first Inductor (Lc), the inductance of said second Inductor (Lin)being larger than the inductance of said first Inductor (Lc), forallowing said second Inductor (Lin) to generate the current required fordriving said PRC; e) a first controllable switch (Q12), a first contactof which being coupled to said first contact of said capacitor, and asecond contact of which being coupled to ground; f) a secondcontrollable switch (Q13), a first contact of which being coupled tosaid second contact of said capacitor, and a second contact of whichbeing coupled to said ground; and g) a Soft Switching Controller (SSC),for soft switching said second and third switches (Q12,Q13), the inputof said SSC being fed with a signal representing the instantaneousmagnitude of the signal at the second contact of said second Inductor(Lin), said SSC generates two complementary trains of digital signal,one of said trains being fed to an input terminal of said second switch(Q12) and the second train being fed to an input terminal of said thirdswitch (Q13), for causing them to alternately switch from conductive tonon-conductive state in synchronization with the instants at which saidinstantaneous magnitude reaches essentially a zero value, only oneswitch being in its conductive state at a given time.
 18. Apparatusaccording to claim 1, in which the high-frequency AC current source isimplemented by utilizing an input circuitry in a Flyback configuration,said circuitry being placed between a DC voltage source and the currentsplitting inductor, comprising: a) a transformer, the primary side ofwhich being an input inductor (L1), a first contact of which could beconnected to a DC power source, and the secondary side of which beingthe current splitting inductor; and b) a controllable switch (Q14), afirst contact of which being coupled to a second contact of said inputinductor (L1), and a second contact of which being coupled to ground,said controllable switch (Q14) causes said input inductor (L1) to storeenergy whenever said controllable switch being in its conductive state,and to forward at least some of the stored energy to said currentsplitting inductor whenever said controllable switch is in itsnon-conductive state.